Thank you very much for this topic, really helpful.
I received my base M5 MacBook Air yesterday, but was surprised to find out my LG 40WP95C-W doesn't support my preferred 3840x1680 HiDPI layout, even though it worked perfectly on my M1 Pro and M2 Pro machines.
The biggest reason I went for the base M5 this time is Apple removed the previous hardware limitation on rendering pixel width (capped at 6016 pixels) with M4. But unfortunately there still appears to be a software enforced limit that prevents higher HiDPI modes unless the system detects a sufficiently demanding display.
To summarize:
I have also a 4K@240Hz display, and on the M5, 3840x2160 runs in standard DPI mode. This is understandable since it's also the native resolution, and macOS may skip HiDPI in such cases.
Therefore, the only way to trigger higher rendering width on base M4/M5 is by connecting either an 8K@60Hz or 5K@120Hz display. This explains why the Dell U4025QW works while the LG 40WP95C-W does not.
While I disagree with this logic, it does make sense when the full context is considered.
Now, a few questions out of curiosity:
I received my base M5 MacBook Air yesterday, but was surprised to find out my LG 40WP95C-W doesn't support my preferred 3840x1680 HiDPI layout, even though it worked perfectly on my M1 Pro and M2 Pro machines.
The biggest reason I went for the base M5 this time is Apple removed the previous hardware limitation on rendering pixel width (capped at 6016 pixels) with M4. But unfortunately there still appears to be a software enforced limit that prevents higher HiDPI modes unless the system detects a sufficiently demanding display.
To summarize:
- Apple engineers seem to believe it's a good idea to allocate display rendering resources conservatively, likely to preserve bandwidth for potential second external display.
- On base M4/M5, the initial rendering width is capped at 6016 pixels (3008 HiDPI).
- On M4/M5 Pro and Max, the limit is higher, 6720 pixels (3360 HiDPI).
- This allocation can increase only if macOS determines no additional displays can be connected.
- Base M4/M5 can drive only one external display at 8K@60Hz, 5K@120Hz, or 4K@240Hz. This likely defines the threshold for "high-demand" detection.
I have also a 4K@240Hz display, and on the M5, 3840x2160 runs in standard DPI mode. This is understandable since it's also the native resolution, and macOS may skip HiDPI in such cases.
Therefore, the only way to trigger higher rendering width on base M4/M5 is by connecting either an 8K@60Hz or 5K@120Hz display. This explains why the Dell U4025QW works while the LG 40WP95C-W does not.
While I disagree with this logic, it does make sense when the full context is considered.
Now, a few questions out of curiosity:
- LG's new 40U990A-W supports 5120×2160@120Hz. Has anyone tested whether this enables 3840x1680 HiDPI on base M4/M5?
- Apple's new Studio Display XDR runs 5K@120Hz. Would it support 3840+ HiDPI?
- For M4/M5 Pro and Max, since they can support two 5K@120Hz displays, what triggers the system to allocate higher rendering width when only one display is connected? Does macOS still enable 3840+ HiDPI even if a second high-demand display is theoretically possible? Maybe you are never going to have that option, because the chip is too powerful.
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